work around linux bug in readlink syscall with zero buffer size

linux fails with EINVAL when a zero buffer size is passed to the
syscall. this is non-conforming because POSIX already defines EINVAL
with a significantly different meaning: the target is not a symlink.

since the request is semantically valid, patch it up by using a dummy
buffer of length one, and truncating the return value to zero if it
succeeds.

src/unistd/readlink.c

@@ -4,9 +4,16 @@
 
 ssize_t readlink(const char *restrict path, char *restrict buf, size_t bufsize)
 {
+	char dummy[1];
+	if (!bufsize) {
+		buf = dummy;
+		bufsize = 1;
+	}
 #ifdef SYS_readlink
-	return syscall(SYS_readlink, path, buf, bufsize);
+	int r = __syscall(SYS_readlink, path, buf, bufsize);
 #else
-	return syscall(SYS_readlinkat, AT_FDCWD, path, buf, bufsize);
+	int r = __syscall(SYS_readlinkat, AT_FDCWD, path, buf, bufsize);
 #endif
+	if (buf == dummy && r > 0) r = 0;
+	return __syscall_ret(r);
 }

src/unistd/readlinkat.c

@@ -3,5 +3,12 @@
 
 ssize_t readlinkat(int fd, const char *restrict path, char *restrict buf, size_t bufsize)
 {
-	return syscall(SYS_readlinkat, fd, path, buf, bufsize);
+	char dummy[1];
+	if (!bufsize) {
+		buf = dummy;
+		bufsize = 1;
+	}
+	int r = __syscall(SYS_readlinkat, fd, path, buf, bufsize);
+	if (buf == dummy && r > 0) r = 0;
+	return __syscall_ret(r);
 }